1. Field of the Invention
The invention relates to a body frame of a straddle-type vehicle such as a motorcycle.
2. Description of Related Art
A rear frame of a motorcycle is conventionally formed from a seat rail extending substantially horizontally and rearward from an upper rear part of a main frame, and a backstay extending obliquely upward and rearward from a lower rear part of the main frame. An upper end of the backstay faces and is welded to a rear end of the seat rail. See, for example, JP-A-H05-262269.
The area of welding of the backstay and seat rail is preferably large to improve the strength of the connection. Thus, predetermined areas of the backstay and the seat rail facing each other along a longitudinal direction are welded together. As shown in FIG. 7, a backstay 100 and a seat rail 101 are conventionally formed from circular pipes. A sideward opening angle between the circular pipes is large, and a welding torch can easily approach a center part of the facing areas. However, when a square pipe is used for backstay 100 or seat rail 101, as shown in FIG. 8, the sideward opening angle between backstay 100 and seat rail 101 is small and a welding torch cannot easily enter. Thus, the conventional connection form cannot easily accommodate a design using a circular and a square pipe.